[jhb_airlines] Re: Another one for Bones...

  • From: "Bones" <bones@xxxxxxx>
  • To: <jhb_airlines@xxxxxxxxxxxxx>
  • Date: Wed, 23 Nov 2005 13:21:45 -0000

I'll have a look at the approach plate and throw some ideas around for a

SDF is a Step Down Fix, so called because a true descent profile isn't
really possible with an NDB approach. The best you can do is work out a
nominal descent rate based on you speed but this can be a tricky calculation
with any wind effect. An SDF is an alternative way of looking at things and
simply means that at a certain point you can descend to the SDF without
worrying about descent rates. You can find SDF's in other approaches too
(like a VOR) if there is high ground on the approach. In this case you
descend to an initial SDF until a certain distance from touchdown after
which the descent can begin again - see EGNS 08 approach.

The problem with talking about the approach procedure is that it can make a
difference whether the aircraft is fitted with a simple ADF display or, as
you mention, has an RMI. The latter are a lot better as they take all the
mental calculations out of the equation - your dial is always showing a QDM
to the beacon.

The other point to raise about NDB procedures is that they rarely line you
up with the runway. The two major reasons for this are that the NDB itself
is located well off the runway centreline and (in a real aircraft anyway)
gyro drift can mean you are intercepting the wrong inbound direction.

Let's have a look at the plate and get back to you..


-----Original Message-----
From: jhb_airlines-bounce@xxxxxxxxxxxxx
[mailto:jhb_airlines-bounce@xxxxxxxxxxxxx] On Behalf Of Peter Dodds
Sent: 23 November 2005 12:05
To: jhb_airlines@xxxxxxxxxxxxx
Cc: pdodds@xxxxxxxxxxxx
Subject: [jhb_airlines] Another one for Bones...

Last night I was flying Garry Blaisdells Twin Otter into
Gloucestershire with VATSIM, and with the poor weather was
obliged to fly the NDB/DME procedure to 09.  Whilst in flight, I
downloaded the plate and with little time to study it, although I
pegged the DME arc inbound from the north, I botched the final
turn and trying to get back to the correct QDM, missed the final
aproach fix at 3DME.  (It seems to be called the SDF now, not
FAF - what is SDF?). I was able to complete the approach visually
fortunately, because, although viz was only just about 3 miles,
FS obligingly shows the PAPI lights through the fog/mist!

The ADF instrument in the Twin Otter has its compass bezel slaved
to the HSI, making it a Radio Magnetic Indicator (RMI) I believe,
not an ADF. Now I think that this affects the instrument
"picture" you look for at the point where you start the final
turn at QDM108degrees/8DME. So what threw me last night was that
I had forgotten that the ADF needle remains horizontal in the DME
arc, its the bezel that turns - right up to the moment of the
final turn. What I was looking for was the needle pointing 14
degrees "up" from horizontal to turn from a QDM of 108 to the
final approach QDM of 094, when in fact I think should have
turned when the instrument bezel had turned so that the still
horizontal needle was pointing to 108 degrees.

John, could you do one of your excellent tutorials on flying this
procedure?  Am I right?  I tried it again this morning and got it
roughly right, but when I came out of cloud at 1600 feet, I was
slightly off line.


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